Lithium-ion secondary batteries with high energy density have been used as power sources for portable electronics such as a mobile phone and a lap-top computer.
An electrode material of a lithium-ion secondary battery includes a positive electrode material, a separator, and a negative electrode material. Regarding the positive electrode material, an aluminum alloy foil has been used as a current collector, having excellent electrical conductivity and less heat generation without affecting electrical efficiency of a secondary battery. The positive electrode material comprises a current collector and an active material layer. Specifically, the positive electrode material can be obtained by first forming the active material layer by applying the active material (including material, hereinafter the same shall apply) having a lithium-containing metal oxide such as LiCoO2 as a chief component on one or both surface of an aluminum alloy foil followed by drying, and then performing compression forming to the active material layer using a pressing machine (hereinafter, this step of compression forming is referred to as press working). Lithium ion secondary battery can be obtained in the following manner. The positive electrode material thus prepared, a separator, and a negative electrode material are stacked, and then the resulting stack is wound. After performing a shaping process to put it in a desired shape, it is encased with an electrolyte solution.
An aluminum alloy foil used for a positive electrode material of a lithium-ion secondary battery has problems that ruptures occur at a bending portion during winding. Thus, high strength is required. In the drying step after the coating of the active material, heat treatment is carried out at about 100 to 200° C. Accordingly, strength of aluminum is decreased by such heating, making the aluminum likely to generate center buckle during press working. This induces wrinkles during winding, which reduces adhesion between the active material and the aluminum alloy foil. In addition, a rupture is likely to occur when slitting in the post-process. In particular, the decrease in the adhesion between the active material and the surface of the aluminum alloy foil would facilitate peeling of the active material during battery use, when charge and discharge is repeated. Therefore, there is a problem that the capacity of the battery would decrease.
Patent Literature 1 discloses an aluminum alloy foil for an electrode current collector of a battery, tensile strength of the bare foil being 220 to 270 MPa. Patent Literature 2 discloses an aluminum alloy foil for an electrode current collector of a battery, tensile strength of the bare foil being 220 MPa or higher. However, neither Patent Literature 1 nor Patent Literature 2 discloses of the strength of a foil after heating. In general, after the foil for lithium ion battery electrode is prepared, an active material layer is further formed. Here, in such case, heat treatment is carried out in order to dry the active material layer. That is, even when the tensile strength of foil for lithium ion battery electrode is sufficient immediately after its preparation, the foil is affected by the heat treatment. When the foil is used as an electrode, the tensile strength of the foil being decreased is problematic regarding the strength. Neither one of the aluminum alloy foils disclosed in Patent Literature 1 nor Patent Literature 2 can solve the problem of such decrease in strength.